How can I change my old low level toilet to a new close coupled toilet without having a 50mm gap behind the cistern. This is a question we have had hundreds and hundreds of times here at DIY Doctor and there are two possible answers.
You either move all of the outgoing pipework in your bathroom wall or floor, or you make a simple box which goes on the wall behind the new close coupled cistern. The new cistern is then fixed to this box which can be tiled or painted depending on what mood you are in!
Even if the bathroom walls are tiled already it is easy, using strong construction adhesive such as Gripfil (search our tool store for this) to stick the required timbers on the wall and then tack some plasterboard to them when the adhesive is dry. Make sure, as in the diagram below, that you insert a timber cross member at the height and level of the screw holes which secure the cistern to the wall.
The reason a low level toilet does not often exchange easily for a close coupled unit is because 99% of the time a low level unit has a waste outlet which goes down into the floor. If the low level unit you have has a waste outlet which goes directly through the wall or turns at a right angle out of the back of the toilet to go right or left away from the toilet, then you should have no problem replacing it with a close coupled toilet. If your low level unit goes into the floor, the floor outlet is too far away from the wall to allow an easy exchange with a close coupled unit. You can either dig up the floor and install another pipe, or continue with the box method described above.